equal_range in C++

std::equal_range is used to find the sub-range within a given range [first, last) that has all the elements equivalent to a given value. It returns the initial and the final bound of such a sub-range.

This function requires the range to be either sorted or partitioned according to some condition such that all the elements for which the condition evaluates to true are to the left of the given value and rest all are to its right.

It can be used in two ways as shown below:

1. Comparing elements using <:

   Syntax:

   Template
   pair 
       equal_range (ForwardIterator first, ForwardIterator last, const T& val);
   
   first: Forward iterator to the first element in the range.
   last: Forward iterator to the last element in the range.
   val: Value of the subrange to search for in the range.
   
   Return Value: It returns a pair object, whose member pair::first 
   is an iterator to the lower bound of the subrange of equivalent 
   values, and pair::second its upper bound.
   If there is no element equivalent to val, then both first and 
   second points to the nearest element greater than val, or if val is
   greater than any other value, then both of them point to last.
   

   // C++ program to demonstrate the use of std::equal_range
   #include <iostream>
   #include <vector>
   #include <algorithm>
   using namespace std;
   int main()
   {
       vector<int> v = { 10, 10, 30, 30, 30, 100, 10,
                         300, 300, 70, 70, 80 };
   
       // Declaring an iterator to store the
       // return value of std::equal_range
       std::pair<std::vector<int>::iterator,
                 std::vector<int>::iterator> ip;
   
       // Sorting the vector v
       sort(v.begin(), v.end());
       // v becomes 10 10 10 30 30 30 70 70 80 100 300 300
   
       // Using std::equal_range and comparing the elements
       // with 30
       ip = std::equal_range(v.begin(), v.begin() + 12, 30);
   
       // Displaying the subrange bounds
       cout << "30 is present in the sorted vector from index "
            << (ip.first - v.begin()) << " till "
            << (ip.second - v.begin());
   
       return 0;
   }

   Output:

   30 is present in the sorted vector from index 3 till 6
   

   Explanation: After sorting the vector v1, we checked for the bounds within which 30 is present, i.e., from index 3 till index 6.

2. By comparing using a pre-defined function:

   Syntax:

    pair 
       equal_range (ForwardIterator first, ForwardIterator last, 
                    const T& val, Compare comp);
   
   Here, first, last and val are the same as previous case.
   
   comp: Binary function that accepts two arguments of the type 
   pointed by ForwardIterator (and of type T), and returns a
   value convertible to bool. The value returned indicates 
   whether the first argument is considered to go before the
   second. 
   The function shall not modify any of its arguments.
   This can either be a function pointer or a function object.
   
   Return Value: It returns a pair object, whose member 
   pair::first is an iterator to the lower bound of the subrange 
   of equivalent values, and pair::second its upper bound. 
   If there is no element equivalent to val, then both first and
   second point to the nearest element greater than val,
   or if val is greater than any other value, then both
   of them point to last.
   

   // C++ program to demonstrate the use of std::equal_range
   #include <iostream>
   #include <algorithm>
   #include <string>
   #include <vector>
   #include <functional>
   using namespace std;
   
   // Defining the BinaryFunction
   bool comp(int a, int b)
   {
       return (a > b);
   }
   int main()
   {
       vector<int> v = { 10, 10, 30, 30, 30, 100, 10,
                         300, 300, 70, 70, 80 };
   
       // Declaring an iterator to store the
       // return value of std::equal_range
       std::pair<std::vector<int>::iterator,
                 std::vector<int>::iterator> ip;
   
       // Sorting the vector v in descending order
       sort(v.begin(), v.end(), greater<int>());
       // v becomes 300 300 100 80 70 70 30 30 30 10 10 10
   
       // Using std::equal_range and comparing the elements
       // with 10
       ip = std::equal_range(v.begin(), v.begin() + 12, 10, comp);
   
       // Displaying the subrange bounds
       cout << "10 is present in the sorted vector from index "
            << (ip.first - v.begin()) << " till "
            << (ip.second - v.begin());
   
       return 0;
   }

   Output:

   10 is present in the sorted vector from index 9 till 12
   

Where can it be used ?

1. std::lower_bound and std::upper_bound at one place: This function can be used if we want to use both std::lower_bound and std::upper_bound at the same time, as its first pointer will be same as std::lower_bound and its second pointer will be same as std::upper_bound. So, there is no use of separately using them, if we have std::equal_range.

   // C++ program to demonstrate the use of std::equal_range
   #include <iostream>
   #include <vector>
   #include <algorithm>
   using namespace std;
   int main()
   {
       vector<int> v = { 1, 2, 3, 4, 5, 5, 6, 7 };
   
       // Declaring an iterator to store the
       // return value of std::equal_range
       std::pair<std::vector<int>::iterator,
                 std::vector<int>::iterator> ip;
   
       // Using std::equal_range and comparing the elements
       // with 5
       ip = std::equal_range(v.begin(), v.end(), 5);
   
       // Displaying the subrange bounds
       cout << "std::lower_bound should be equal to "
            << (ip.first - v.begin()) << " and std::upper_bound "
            << "should be equal to " << (ip.second - v.begin());
   
       vector<int>::iterator i1, i2;
   
       // Using std::lower_bound
       i1 = std::lower_bound(v.begin(), v.end(), 5);
       cout << "\nstd::lower_bound is = " << (i1 - v.begin());
   
       // Using std::upper_bound
       i2 = std::upper_bound(v.begin(), v.end(), 5);
       cout << "\nstd::upper_bound is = " << (i2 - v.begin());
   
       return 0;
   }

   Output:

   std::lower_bound should be equal to 4 and 
   std::upper_bound should be equal to 6
   std::lower_bound is = 4
   std::upper_bound is = 6